This invention relates to semiconductor materials and particularly to encapsulated quantum sized doped semiconductor materials and methodology for manufacturing these particles by precipitation. These particles may be described as doped nanocrystals (i.e. crystallites of less than 100 .ANG.) which exhibit quantum effects.
This application is related to application Ser. No. 08/050,693 entitled "Method of Manufacturing Quantum Sized Doped Semiconductor Particles" filed Apr. 20, 1993 which is directed to a heterogeneous process for manufacturing nanoparticles within a polymer matrix and the disclosure of which is hereby incorporated by reference.
By making semiconductor particles small enough to show quantum confinement effects and doping them with a luminescent activator element, new optical properties are created which differ from those of chemically identical bulk material and from the quantum confined host material alone. An activator doped and quantum confined host material was made which demonstrates a blue shift (shorter wavelengths) in the excitation wavelengths of the activator. These systems also display a dramatic decrease (&gt;10.sup.5) in the time for luminescence decay with an efficiency and brightness comparable to bulk ZnS:Mn phosphors. These new material characteristics indicate a fundamental change in the optical properties which results from the quantum confinement. Other methods to produce this material and other unique properties, such as reduced excitation voltages for flat panel cathode ray tubes, are also possible.
It has been recognized that when the radius of a semiconductor crystallite is near that of the Bohr radius of the exciton, there is a quantum size effect and its electronic properties change (Y. Wang and N. Herron, "Nanometer-Sized Semiconductor Clusters: Materials Synthesis, Quantum Size Effects and Photophysical Properties", J. Phys. Chem. 95 525, 1991). This had been observed as a blue shift (shift to shorter wavelengths) in the optical bandgap for quantum sized ZnS particles in solution (H. Weller, U. Koch, M. Guitierrez and A. Henglein, "Photochemistry of Colloidal Metal Sulfides. Absorption and Fluorescence of Extremely Small ZnS Particles (The World of Neglected Dimensions)", Ber. Bunsenges. Phys. Chem. 88 649, 1984). Most of the II-VI and some III-V and group IV semiconductors have been prepared as quantum sized particles and demonstrate quantum size effects in their physical properties. The size at which the particles demonstrate changes in their bandgap from the quantum size effects vary with the intrinsic electronic structure of the compound but typically appear when below 100 .ANG. in diameter. To exhibit quantum size effects it is also necessary for the particles to remain isolated from one another, if allowed to aggregate the material exhibits bulk properties despite the small size of the individual particles.
Quantum confinement effects were first described with semiconductors prepared by precipitation. The precipitation of particles from chemical solutions is induced either by the creation of a new phase in a chemical reaction or by the supersaturation of a soluble phase. This is the most basic of materials processing techniques. To control the nuclei size on the basis of thermodynamics, one is balancing the free energy decrease from the formation of the lower energy phase vs. the increase in free energy from the new surface formed. To make the precipitation nuclei small, one must choose a chemical system where the decrease in free energy from forming the new phase is large --i.e. a highly reactive chemistry, the precipitation product should have a low solubility in the solvent, and the reaction should be spontaneous to limit diffusional growth of the particles after nucleation. Chemistry is not the only issue, physical conditions like concentration of reactants also affects the nuclei number density and thus controls the agglomeration and the growth of particles after precipitation.
Most homogeneous precipitation of nanometer sized undoped particles has used an aqueous salt chemistry (e.g. Zn(ClO.sub.4)+NaHS.fwdarw.ZnS in water). The present methodology uses an organometallic chemistry in a hydrocarbon solvent. See, e.g., Johnson et al (C. E. Johnson, D. K. Hickey, and D. C. Harris, "Synthesis of Metal Sulfide Powders From Organometallics," Mat. Res. Soc. Symp. Proc. Vol 73,785-789, 1986) wherein 0.1 .mu.m undoped ZnS particles (not quantum sized) were produced in organic solvents. The attraction of this chemistry is that the powders are highly crystalline and had very low residual organic content (&lt;60 ppm alkane). In the present development, by controlling the physical process parameters, a similar chemistry can be used to make quantum sized particles (&lt;50 .ANG.). However the particles must also be doped.
The doping of semiconductor powders with a manganese is usually accomplished by the thermal diffusion of Mn from a salt or carbonate at 1100.degree. C. Thermal diffusion is impractical for nanometer sized particles because they melt and sinter at extremely low temperatures (e.g. 18 .ANG. CdS particles melt at 200.degree. C. vs 1403.degree. C. in the bulk material). Physical and morphological changes at elevated temperatures will ruin the quantum confinement characteristics of this system. Thus the material must be doped during the particle formation by a chemical process.
The manganese ion is large and multivalent and reacts readily to form new compounds in solution --this makes incorporating Mn during ZnS precipitation difficult. To solubilize the metal in an organic solvent it is necessary to use an organic chelating ligand, but most Mn organometallics are unstable and either polymerize or precipitate a separate Mn inorganic phase which precludes the doping of ZnS. The present work utilizes the formation of a metastable intermediate organometallic compound which is compatible with the present ZnS forming reaction to provide the Mn. This is the first known use of a chemical doping process in the precipitation of nanometer sized ZnS particles.
The reaction to form the manganese organometallic is a synthesis technique known as a Grignard reaction which uses an organomagnesium halide in an exchange reaction with a metal salt. The reaction used herein is: EQU 2C.sub.2 H.sub.5 MgCl+MnCl.sub.2 .fwdarw.(C.sub.2 H.sub.5).sub.2 Mn+2MgCl.sub.2
with tetrahydrofuran as a solvent. There are many other possible Grignard reaction chemistries that will provide a manganese organometallic. Since most manganese reaction products are unstable, they must be used immediately in the ZnS synthesis reaction to provide the desired Mn.
To maintain the separation for quantum confinement in precipitated particles, it is also necessary to add a material which coats the surface of the particles and provides a barrier to agglomeration. These molecules are commonly referred to as surface active agents--surfactants. The requirements of a surfactant for the present system are straightforward:
1. must be soluble in hydrocarbon solvents PA1 2. must not participate in the chemical reactions to dope or form the ZnS PA1 3. must have an ultraviolet absorption below the absorption edge of the host matrix (e.g. ZnS). This requirement may be ignored for long wavelength or non-optical applications. PA1 Luminescent phosphors for use in cathode ray tubes and lights. PA1 Thin films for electroluminescent displays. PA1 Lasing phosphors. PA1 The use of luminescent activators and magnetic particles for magneto-optical recording and displays. PA1 Lower voltage phosphors for flat cathode ray tubes. PA1 Markers for medical diagnosis.
It is the ultraviolet absorbance which excludes most known surfactants, nevertheless it is believed that there are many molecules which could suffice. The surfactant used in this development was poly(methyl methacrylate) (PMMA) and its monomer, methacrylic acid. PMMA has been studied as a surfactant and has demonstrated both physical adsorption (thermodynamic driven) and chemical adsorption (hydrogen bonding with C.dbd.O group in polymer) onto oxide surfaces.
The nanocrystals (doped particles &lt;100 .ANG.) produced by the present invention have a luminescent efficiency which is high for films prepared at room temperature. Normally, bulk ZnS:Mn used in electroluminescent devices yield high efficiency when prepared above temperatures of 350.degree. C. For powder phosphors, this temperature is frequently as high as 1000.degree. C. The new doped nanocrystals also emit light significantly faster (shorter luminescent decay time) than that observed with corresponding bulk material. This faster luminescent decay time in a nanocrystal provides advantage over bulk material for application where speed is important, i.e. faster phosphors for next generation TV's and displays.
Possible applications for new materials based on the concepts and materials described in this application include: